Resin particles with high bulk density are desirable for efficient plant operations and high throughput. Resin particles should also have regular shape and narrow particle size distribution for good flowability during resin extrusion. They are also essential to produce regular shaped copolymer products with uniform rubber dispersion. Shape regularity, size distribution and bulk density of polymer/polyolefin resin is usually related to the shape and size of the catalyst particles as well as of the solid reactants from which the catalyst particles are synthesized. Usually solid magnesium containing materials such as magnesium alkoxides and magnesium halides are used as precursors.
WO 2005/044873 discloses a method for synthesizing spherical magnesium alkoxide particles by reacting magnesium with an alcohol mixture at a temperature below the boiling point of the mixture. The end product obtained is a mixture of magnesium ethanolate, magnesium methanolate and magnesium isopropanolate. The spherical magnesium alkoxide particles synthesized by the method have lower mean particle diameter (<40 μm) and are fragile and do not retain their morphology or particle size during the synthesis of Ziegler Natta procatalyst, especially when the procatalyst synthesis is carried out on a plant scale using high attrition of process. Further, the resin produced using this method exhibits low resin bulk density which can hamper the plant throughput.
In another approach, magnesium halide alcoholate formed using magnesium dichloride and aliphatic alcohol can also be used as precursor for making solid catalyst comprising titanium compound such as titanium tetrahalide (U.S. Pat. No. 4,948,770).
US20080262178 A1 discloses spherical magnesium alcoholate precursor particles prepared using mixed alcohols in presence of halogen and/or an optionally organic halogen compound. Catalyst comprising such precursor is used for gas phase polymerization of polypropylene with low fines and higher bulk density. However in US20080262178 A1 the reaction temperature used for precursor preparation is below boiling point of alcohols, which results in longer batch time. The particle diameter obtained is preferably between 10 and 50 μm to retain the sphericity of catalyst. The gas phase polypropylene produced using the precursor catalyst has bulk density of 0.46 g/cc with low fines.
U.S. Pat. No. 4,663,299 A teaches a process for preparing spherical particles of a compound with the general formula Mg(OR)2-a (OR′)a. In the process R is an alkyl group with from 2 to 8 carbon atoms, R′ is an alkyl group with from 1 to 7 carbon atoms and ‘a’ is a number of from 0 to 0.5. The process comprises preparing a homogeneous solution of a compound Mg(OR)2 in an aliphatic alcohol with the general formula R′OH, spray-drying the solution at a temperature of from 15° to 85° C., suspending the spray-dried solid particles in an aliphatic alcohol with the formula ROH in which R is the same alkyl group as present in the said compound Mg(OR)2, heating the suspension in order to remove an alcohol with the formula R′OH from the liquid phase by distillation and separating the solid particles from the liquid phase. U.S. Pat. No. 4,663,299 A teaches magnesium alkoxide precursor using mixed alcohol (ethanol and methanol) involving spray drying and distilling off one alcohol during preparation. This is used to prepare a catalyst used to prepare polyolefins in high bulk density with lower fines. However, this results in having additional step of drying.
WO 2009/084799 A1 provides a method of preparation of dialkoxy magnesium that is used as a support for an olefin polymerization catalyst for polyolefin preparation. Specifically, it provides a method of preparation of a support for an olefin polymerization catalyst, which comprises preparation of dialkoxy magnesium by reacting magnesium metal with alcohol in the presence of a reaction initiator, wherein bromine is used as the reaction initiator so as to obtain spherical dialkoxy magnesium. Dialkoxy magnesium support of this document is prepared by continuous addition of magnesium and alcohol into the reactor; and bromine as initiator; resulting in low bulk density.
U.S. Pat. No. 5,223,466 A describes a process for making a magnesium-containing supported titanium-containing catalyst, alpha-olefin polymerization or co-polymerization catalyst or catalyst component having a large particle size, semi-spherical particle shape and high resistance to attrition, but precursor syntheses process employs dissolving and re-precipitation, which makes the process very expensive.
U.S. Pat. No. 4,300,649 describes a solid catalyst component (procatalyst) obtained by heating soluble magnesium compound such as magnesium chloride with an alcohol of at least 4 carbon atoms, in the presence of an ester to produce a solution which is added to titanium tetrachloride and an electron donor to form a solid catalyst. This approach can give good morphology under very stringent and controlled conditions but catalyst yield per unit mass of precursor consumption is low and the cost of catalyst production is very high.
In another variant as explained in U.S. Pat. No. 5,066,737, a complex-magnesium-containing and titanium-containing improved morphology of solid olefin polymerization procatalyst precursor is produced by reaction of magnesium alkoxide, titanium tetraalkoxide and a phenolic compound followed by removal of alkanol. Conversion of this solid to an olefin polymerization procatalyst and the procatalyst to an olefin polymerization catalyst results in a catalyst which is used to polymerize lower alpha-olefin monomers. The polymer product is obtained in good yield and has good properties including a relatively high bulk density and a relatively narrow particle size distribution. However, synthesis of this type of precursor is rather complicated and involves several steps. As the number of stages increase, the tendency of particle breakage due to mechanical attrition also increases.
None of the prior art discloses preparation of a catalyst system, which when used in commercial polymerization can yield resin particles with particle size >1 mm. If such resin, without fines, is directly produced from the reactor then additional extrusion is not required for converting resin to pallets post polymerization. It is a complex process to synthesize catalysts which maintain their sphericity with higher sized precursor particles because bigger precursor particles generate fines and broken particles, once they are put in high attrition catalyst synthesis process.
The above-mentioned prior art processes suffer from several drawbacks. In all these processes the precursor, procatalyst thereof and polyolefin resin produced do not have the desired particle characteristics, or are produced in a low yield or involve series of reaction stages. Therefore, there is a need to develop a process, which overcomes one or more of the above limitations. Precursors of different particle size are required to produce procatalysts of different particle size. The present invention provides an improved precursor and procatalyst thereof and efficient process for preparing them, which improves polyolefin resin production.